It is desirable to include overload protection devices such as fuses in combination with electrical devices which upon failure do not draw sufficient current to melt or blow an external fuse. Two fuse elements, one for each terminal of a plug are often required inasmuch as most plugs are reversible and it is impractical to determine which terminal is the primary current carrier and should therefore be fused.
Conventional overload protection fuses are expensive and contribute substantially to the cost of product manufacture.
Various attempts have been made to produce inexpensive yet reliable fuses. The most common of these is the auto glass replaceable fuse in which a fuse wire is enclosed in and extends through a glass tube. The protruding ends of the wire are electrically secured to a pair of conductive end caps for subsequent external connection. Such constructions are expensive and bulky and afford only high over current protection.
A layered fuse construction has been employed as an inexpensive alternative to the glass envelope construction. In a typical construction, a conductive strip is superimposed on a substrate and a metal backing layer. These constructions are inexpensive and permit direct access to the fuse conductor, but do not provide low over current protection for lower power devices. The fuse substrate does not contribute to the protective action of the fuse but serves only as base on which to support the conductive strip. The protective action of these fuses is solely attributable to melting of the conductive strip in response to excessive current therethrough. Teflon coated Kapton and Mylar (tradenames of E. I. du Pont de Nemours & Co.) have been suggested as suitable substrates for the metal strips. These fuses are employed in heavy duty appications and require a rigid metal backing layer to abstract heat from the substrate and prevent heat from building up and causing the conductive strip to prematurely melt.
Other fuses provide both high and low over current protection and employ a conductive strip which is secured to an expandable substrate. In a particular example, a conductive strip is secured to a ceramic substrate. The substrate is a functional part of the fuse and contributes to the fuse protection by expanding and cracking in response to continuous or durational low over current conditions. Fracture of the substrate also breaks and separates the conductive strip secured thereto. Additionally during high overcurrent conditions the conductive strip melts in response to the normal joule or I.sup.2 R heating created by current passing through the conductive strip. Ceramic fuses are unattractive because the substrate layer must be thin and substantially porous in order for it to expansively crack under low over current conditions and consequently the fuse is expensive and fragile. The ceramic fuse is particularly inappropriate for rough service applications such as in an electrical plug.
Oriented plastic sheet material have been used in a fuse as substrate materials as disclosed in copending application Ser. No. 858,940, (now U.S. Pat. No. 4,208,645) assigned to the assignee of the present invention and incorporated herein by reference. While oriented plastic substrate fuses do provide both high and low over current protection, oriented plastic sheets are expensive and difficult to work. Also while it is preferrable that the orientation of the sheet be disposed in relation to the conductive strip. The orientation of the sheet is not readily determinable. Additionally the degree or extent of orientation must be taken in to account as a variable in fuse calibration.